A NOR flash memory device comprises a memory cell adapted to store at least two bits of data. A read operation is performed on the memory cell by generating a reference current with a first magnitude to detect the value of a most significant bit (MSB) and generating the reference current with a second magnitude to detect the value of a least significant bit (LSB). The respective values of the MSBand the LSB are detected by comparing the first and second reference currents to an amount of current flowing through the memory cell during the read operation. The respective magnitudes of the firstand second reference currents are determined by different reference voltages generated by a reference voltage generator.